In modern electric power generation and distribution systems, polyphase alternating current is typically generated and distributed. A number of AC sources producing equal voltages at the same frequencies, at fixed but different phase angles provide the power. In an n-phase system, n voltage sources are connected together. Each voltage source produces a sinusoidally varying voltage of a fixed magnitude. The phase angle associated with each generated voltage varies from the phase angle associated with the voltage from another source by 2.pi./n radians. Current generated by each source may be provided to a single phase load, or to one phase of a polyphase load such as a polyphase motor or transformer.
Conveniently, the n voltage sources may be interconnected to each other at a common point. Modern power distribution systems are typically three phased. In a three-phase system, voltage sources and sinks connected at a common point are said to be connected in a "wye-configuration" or "star configuration". Alternatively, in a three-phased circuit, the voltage sources or sinks may be connected in "delta configuration".
While it is possible to interconnect multiple sources in a poly-phase system in a number of ways, the wye-configuration is generally desirable in three-phase systems. Specifically, for safety and other reasons, it is desirable to electrically connect the polyphase system to ground. Wye-connected source, provide a logical connection point for ground, namely the common (neutral) point of the n voltage sources.
Loads connected to an n-phase system may be chosen such that the net sum of the currents drawn from all sources or "phases" at any time, equals zero. For example, if currents drawn from each voltage source in an n-phase system are equal in magnitude, and displaced in phase by 2.pi./n radians, the phasor sum of the currents drawn from all sources is zero. In the above described wye-configuration, if the net sum of all currents drawn from the phases is zero, the polyphase circuit is said to be "balanced". Of course, if in an otherwise balanced system the current drawn from any of the sources varies, the system will no longer be balanced. Modern poly phase generation and distribution systems are designed and maintained in order to maintain a near balanced system.
In operation, however, a polyphase system is rarely perfectly balanced. While the loads may be chosen to balance the system, the demands on each phase often vary unpredictably with time. Each load may be subject to overvoltage, produced by surges impressed on the distribution system by way of lightning, switching, or the like. Similarly, a load may be prone to operate in resonance, thereby producing an overvoltage. Additionally, harmonics of the base operating frequency of the voltage sources may be present in the system. These harmonics may, for example, result from loads having non-linear voltage/current relationships, such as certain filters or rectifiers. Certain harmonics, such as the third harmonic of three voltage sources delivering current at a fixed (fundamental) frequency and displaced in phase by 2.pi./3 radians, are no longer out of phase. For, example in a three-phase system, generated currents are 2.pi./3 radians out of phase; third harmonics of these currents will have phase differences of 3*2.pi.=2.pi.=0 radians. These harmonics are consequently zero phase harmonics; 6th, 9th, 12th and 15th harmonics will similarly be zero phase harmonics in a three phased system. As currents attributable to their harmonics are in phase, their phasor sum will not equal zero.
The difficulties associated with the overvoltage of loads and zero phase harmonics may be limited by directly grounding the common point of the wye-connected three phase circuit. Thus, in balanced operation, no current will flow from this common point to ground, as this common point remains at or near zero potential in view of the balanced loads. In the event of an overvoltage, the potential difference between this grounded common point and an affected load will be limited to the overvoltage of that load. No other phase of the n-phased load or single phase load will be affected by overvoltage in one of the loads.
On the other hand, in a situation where one of the loads suffers a fault, caused by, for example, machine failure, an excess amount of current is drawn by a single phase of the circuit. This excess current drawn may impact on the current provided to loads by the remaining phases in the circuit. If the common point of the circuit is connected to ground, much of the fault current will flow from or to this ground connection. Similarly, currents attributable to zero phase harmonics will similarly flow from or to this ground connection. However, if the common point of the circuit is directly grounded, as described above, the amount of fault current flowing from ground through the common point to the load is not limited.
One suggested compromise to grounding the common point of the polyphase circuit has been to connect this common point to ground through an electrical impedance. Thus in the event of failure, the current drawn through the common point will be limited by the impedance. The impedance may take the form of an inductive, reactive or resistive load. The insertion of such an impedance may however create other problems, as for example described in U.S. Pat. No. 1,378,577. If the impedance is reactive it may interfere with the proper functioning of electrical equipment connected to the transformer. If the impedance is purely resistive, resistive losses will occur any time the polyphase circuit in not perfectly balanced. As balancing of a polyphase circuit is typically imperfect, the use of a resistive connection between the common point and ground may be the source of significant losses over time.
The present invention attempts to overcome some of the disadvantages of known circuits used to limit fault current in a polyphase circuit.